A3.1 Diversity of Organisms (IB Biology)

A3.1 Diversity of Organisms What is a species? What patterns are seen in the diversity of genomes within and between species? A3.1.1—Variation between organisms as a defining feature of life No two individuals are identical in all their traits. There is the le...

A3.1 Diversity of Organisms What is a species? What patterns are seen in the diversity of genomes within and between species? A3.1.1—Variation between organisms as a defining feature of life No two individuals are identical in all their traits. There is the least variation when two organisms are genetically identical. In humans, monozygotic (identical) twins are formed when a zygote or early-stage embryo divides and develops into two individuals. Despite starting out with the same genes, both twins acquire differences through mutations and because the environment in which they develop is never identical. The patterns of variation are complex and are the basis for naming and classifying organisms. A3.1.2—Species as groups of organisms with shared traits From the 17th century onwards, biologists have used the word “species” for a group of organisms with shared traits. Carl Linnaeus was a pioneer of naming and classifying species based on their physical form, known as morphology. The morphological species concept is the idea that species are a group of organisms that share a particular outer form and inner structure. A3.1.3—Binomial system for naming organisms Scientists use an international system of naming species called the binomial system. coyote = Canis Each species name consists of two words: the genus latrans and the species. A genus is a group of species that have similar traits. The second name is the species or specific name. Rules about binomial nomenclature: 1. The genus name begins with a capital letter. 2. The species name begins with a lowercase letter. 3. In typed or printed text, a binomial is shown in After using the binomial name once, what abbreviation could you use? italics. 4. After a binomial or genus name has been used once in a piece of text, it can be abbreviated to the C. latrans first letter of the genus name with the full species name. A3.1.4—Biological species concept There are many competing definitions of what defines a species. According to the biological species concept, a species is a group of organisms that can breed and produce fertile offspring. Horse 3 is fertile so both Horse 1 and x = Horse 2 are the same species Horse 1 Horse 2 Horse 3 A mule is infertile so therefore horses and x = donkeys are different species horse donkey mule A3.1.4—Biological species concept However, there are challenges to this species concept including: captive lions and tigers in captivity have sometimes hybridized, producing ligers (male lion x female tiger) or tigons (male tiger x female lion) in genera in which speciation is occurring rapidly like certain conifers, there is some interspecific hybridization and it is less easy to identify species liger A3.1.5—Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation A population is a group of organisms of the same species living in the same area at the same time. Two populations that live in different areas can be the same species despite the fact that they rarely interbreed with each other. However, if two populations do not interbreed, they can diverge. Recognizable differences may develop as the species become genetically more different. If differences continue to accumulate, the two populations may eventually become different species. Speciation is the splitting of one species into two or more. It usually happens gradually rather than by a single act, with populations becoming more and more different in their traits. It can therefore be an arbitrary decision whether two populations are regarded as the same or different species. A3.1.5—Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation Different species have different numbers of chromosomes. During evolution, chromosome number can humans decrease if chromosomes fuse, or increase 46 chromosomes if they split. There are also mechanisms that can cause the chromosome number to double. However, these changes are rare and usually there is no change in chromosome number for millions of years. For example, humans have 46 chimpanzees chromosomes and chimpanzees have 48. 48 chromosomes (these are the only two specific numbers you should memorize) A3.1.5—Difficulties distinguishing between populations and species due to divergence of non-interbreeding populations during speciation Diploid (2n) - body cells with two sets of chromosomes, one from each parent Haploid (n) - sex cells with one set of chromosomes which can combine with another haploid cell to make a diploid cell Diploid (body) cells have an even number of chromosomes. This is due to sexual reproduction - a male gamete and a female gamete fuse to make a new organism. Since both the male and female gamete had the same number of chromosomes since they are the same species, the resulting doubling will always be an even number. A3.1.7—Karyotyping and karyograms To study the chromosomes of an organism, scientists look at cells which are currently dividing, stain them, and photograph and arrange the chromosomes digitally in an image called a karyogram. Chromosomes are classified based on three types of difference: 1. banding patterns 2. size 3. position of the centromere which holds paired chromosomes together The characteristic types of chromosome in a species are called the karyotype. An image showing the karyotype is called a karyogram. A3.1.7—Karyotyping and karyograms Human somatic (body) cells have 46 chromosomes. Our closest primate relatives–chimpanzees, gorillas, and orangutans–all have 48. Human chromosome types are numbers from 1 to 22, from largest to smallest. One hypothesis is that human chromosome 2 was formed from the fusion of two chromosomes in a primate ancestor. Figure 11 shows banding patterns of human chromosome 2 compared with chromosomes 12 and 13 from chimpanzees. A3.1.7—Karyotyping and karyograms 1. Compare and contrast human chromosome 2 with the two chimpanzee chromosomes (12 and 13). a. banding pattern of the long arm of chimp 13 is very similar to (much of) the long arm of human 2 b. long arm of chimp 12 and short arm of human 2 have the same banding pattern c. some bands on the short arm of chimp 13 are missing from human 2 d. centromere from chimp 13 is missing from human 2 A3.1.7—Karyotyping and karyograms 2. The ends of chromosomes, called telomeres, have many repeats of the same short DNA sequence. If the fusion hypothesis were true, predict what would be found in the region of the chromosome where the fusion is hypothesized to have occurred. a. either telomere from chimp chromosomes deleted b. or non-functional telomeres still present c. or numbers of repeats reduced A3.1.7—Karyotyping and karyograms 3. Normally a chromosome has just one centromere, but in chromosome 2 there are remnants of a second centromere. Explain this observation. a. centromere from chimp chromosome 12 has become the centromere of human chromosome 2 b. centromere from chimp 13 remains in part as a remnant on human 2 A3.1.7—Karyotyping and karyograms 4. Discuss the strength of the evidence for a fusion of chimp chromosomes in the evolution of chromosome 2 in humans. a. evidence is strong b. considerable similarity in banding/sequences c. very unlikely to be due to chance d. banding patterns from chimp chromosome 12 and 13 both found on human 2 e. remnant of a second centromere is further evidence A3.1.8—Unity and diversity of genomes within species The genome is all the genetic information of an organism, in other words, all of an organism’s DNA. A genome contains functional units called genes. A gene is a length of DNA carrying a sequence of hundreds or thousands of bases. Typically, members of the same species have the same genes, in the same sequence, along each of their chromosomes. A3.1.8—Unity and diversity of genomes within species Diversity within a species is caused by alternative forms of a gene called alleles. Alleles usually only differ from each other by one or a very small number of bases. Sometimes larger sections of a gene become altered, but this usually results in a loss of gene function. Positions in a gene where more than one base may be present are called single-nucleotide polymorphisms, abbreviated to SNPs and pronounced “snips.” SNPs are the main factor in making humans different from each other. Within one individual human there are typically about 4,000-5,000 SNPs out of over 3 billion bases, so only about one base in 650,000 is different from that commonly occurring in humans. A3.1.9—Diversity of eukaryote genomes Genomes vary in overall size, which is determined by the total amount of DNA and measured in base pairs. Large genomes can contain a large amount of non-functional DNA, so they do not necessarily contain more functioning genes than smaller genomes. For example, almost half the human genome consists of transposons, most of which have no known function. Organism Genome Description size/million base pairs Paramecium 27 unicellular tetraurelia organism Apis mellifera 217 honey bee Homo sapiens 3,080 human Pan troglodytes 3,175 chimpanzee Paris japonica 150,000 woodland plant A3.1.9—Diversity of eukaryote genomes Genomes also vary in base sequence. More differences accumulate over time due to mutation. Variation between species is much larger than variation within a species. A3.1.10—Comparison of genome sizes Genome size databases can be used to compare genome sizes between species. Genome size is typically given as nuclear DNA contents of a haploid cell such as a gamete (C-value). Genome size may be given in units of mass (usually picograms; 1pg = 10-12 grams) or in number of base pairs or megabase pairs (1Mbp = 106 base pairs). Use a Genome Size Database to determine the genome size of the following organisms: A3.1.11—Current and potential future uses of whole genome sequencing Whole genome sequencing is determining the entire base sequence of an organism’s DNA. This was first done in the 1990s but was very slow and expensive. Today, the increasing speed and decreasing costs of whole genome sequencing has allowed for the genome of thousands of species of organisms to be sequenced. For example, it cost $100million to sequence one human genome in 2001, but the cost today is less than $1,000. The current uses of whole genome sequencing include: research into evolutionary relationships - comparisons between genomes allow researchers to identify relationships between species and trace the diverging pathways from common ancestors which can make it easier to conserve and protect biodiversity potential future uses include personalized medicine - if individual humans have their genomes sequenced, in the future we may be able to predict health problems and prescribe appropriate drugs and treatments for that individual person A3.1.11—Current and potential future uses of whole genome sequencing  End of SL Content Additional Higher Level Content Past This Point